In a DRAM memory device, data in memory cells must be refreshed at least at a predetermined rate to maintain the data therein. Refreshing of the data is needed because the memory cells are formed from capacitors that leak current (i.e., lose their stored data state) over time. If the time between refreshing (hereinafter “refresh period”) is shorter than needed to maintain the data, then current is unnecessarily consumed. In contrast, if the refresh period is longer than needed, then data may be lost from the memory cells. Accordingly, it is necessary to appropriately control the refresh period of the DRAM memory cells.
The rate at which data is lost from the memory cells can vary based on the temperature of the memory device. Accordingly, a memory device may vary the refresh period of the memory cells responsive to a sensed temperature of the memory device.
FIG. 1 is a graph that illustrates how a refresh period that is needed by a DRAM device can change depending upon its temperature. Referring to FIG. 1, the X-axis represents the temperature of the DRAM device and the Y-axis represents the refresh period in log scale. If the temperature of a DRAM device is about 5° C., the needed refresh period is about 1s, while if the temperature of the DRAM device is about 85° C., the needed refresh period is shortened to about 100 ms. Accordingly, the temperature of the DRAM device can be sensed, and the refresh period can be set longer when the sensed temperature of the DRAM device is low and can be set shorter when the sensed temperature of the DRAM device is high. In FIG. 1, the refresh period is set longer below 45° C. and set shorter above 45° C.
Accordingly, a temperature sensor can be used in a DRAM device to vary the refresh period and associated operations of a controller in the DRAM device, and which may reduce the current consumption of the device.
FIG. 2 illustrates an output signal of a conventional temperature sensor based on sensed temperature. Referring to FIG. 2, when the sensed temperature increases above a predetermined temperature, the temperature sensor outputs a high binary level signal and when the sensed temperature is below the predetermined temperature, the temperature sensor outputs a low binary level signal.
If the temperature sensor changes its output signal at a 1 ms rate and senses the temperature of a device that operates around 50° C. for even a brief period, the output of the temperature sensor can alternate at a 1 ms rate between opposite binary levels (low and high). Such frequent change in the output signal binary level from the temperature sensor may cause a DRAM device, which varies its refresh rate based on the temperature signal, to erroneously change its refresh period and/or may cause other erroneous operation of the DRAM device. For example, the operational speed of the DRAM device may be reduced when a controller in the DRAM device is excessively interrupted by the frequent changes in the temperature signal.